The field of this invention is that of self-regulating electrical resistance heaters and fluid flow sensors and the invention relates more particularly to bodies of ceramic electrical resistance material of positive temperature coefficient of resistivity (PTC) having passages extending through the body which are adapted for heating fluid flowing through the body passages or for sensing change in fluid flow through the passages to display change in resistivity of the body material as an indication of the change in fluid flow.
Ceramic electrical resistance materials of positive temperature coefficient of resistivity which are adapted to display sharply increasing resistivity when heated to a selected temperature are widely used as self-regulating electrical resistance heaters and as thermally-responsive sensors. Such materials are formed in a variety of processes which typically include a first heating step in which precursors of the ceramic material are calcined for producing materials with the desired positive temperature coefficient of resistivity and a second heating step in which the ceramic materials are sintered for forming a body of a desired configuration. It is also well known to form multipassaged bodies of such PTC materials by molding or extruding processes so that the bodies are adapted to pass fluids such as air-fuel mixtures in a carburetor through the body passages in close, heat-transfer relation with the electrical resistance heater material of the body. However, such heater bodies have been difficult and expensive to manufacture particularly in smaller sizes and with irregular shapes and it would be desirable to be able to form such flow-through PTC heaters and sensors with increased porousity in a more economical manner which is more easily adpated to providing desired porousity in a body of small or irregular shape so that the body can be, for example, more easily fitted into conduits and the like for heat exchange with fluids passing in the conduits.